1. Field of the Invention
The present invention relates to sole structures for footwear. The invention concerns, more particularly, a footwear midsole that incorporates a lattice material.
2. Description of Background Art
Conventional articles of athletic footwear include two primary elements, an upper and a sole. The upper is usually formed of leather, synthetic materials, or a combination thereof and comfortably secures the footwear to the foot while providing ventilation and protection from the elements. The sole often incorporates multiple layers that are conventionally referred to as an insole, midsole, and outsole. The insole is a thin, cushioning member located adjacent to the foot that enhances footwear comfort. The midsole forms the middle layer of the sole and serves a variety of purposes that include controlling potentially harmful foot motions, such as over pronation; shielding the foot from excessive ground reaction forces; and beneficially utilizing such ground reaction forces for higher jumping or more efficient toe-off. In order to achieve these purposes, the midsole may have a variety of configurations, as discussed in greater detail below. The outsole forms the ground-contacting element of footwear and is usually fashioned from a durable, wear resistant material that includes texturing to improve traction.
The primary element of a conventional midsole is a resilient, polymer foam material, such as polyurethane or ethylvinylacetate, that extends throughout the length of the footwear and is structured to have greater thickness in the heel region of the footwear. The properties of the foam midsole are primarily dependent upon factors that include the dimensional configuration of the midsole, the material selected for the polymer foam, and the density of the midsole material. By varying these factors throughout the midsole, the relative stiffness, degree of ground reaction force attenuation, and vibrational frequency may be altered to meet the specific demands of the activity for which the footwear is intended to be used.
In general, stiffness, ground reaction force attenuation, and vibrational frequency are related properties of a foam midsole. An increase in stiffness, for example, results in a decrease in the degree of ground reaction force attenuation and an increase in vibrational frequency of the midsole. Accordingly, relatively compliant foam midsoles have a high degree of ground reaction force attenuation and low vibrational frequency. Although high ground reaction force attenuation is a desirable quality for footwear, compliant midsoles often return little energy, thereby imparting a non-energetic feel to the footwear. Consequently, footwear manufacturers attempt to design midsoles so as to achieve a suitable balance between stiffness and degree of ground reaction force attenuation.
Conventional foam midsoles, which have a suitable stiffness/ground reaction force attenuation balance, typically vibrate at frequencies between 10 and 20 Hertz. The vibrational frequency of foam midsoles has an effect upon joints, including the ankles and knees. In general, higher frequencies, particularly above 30 Hertz, induce greater stresses in the joints whereas lower frequencies induce lesser stresses. Accordingly, the vibrational frequency of a foam midsole is generally considered when providing a balance between stiffness and ground reaction force attenuation.
In addition to foam materials, conventional midsoles may include, for example, stability devices that resist over-pronation and moderators that distribute ground reaction forces. The use of foam midsole materials in athletic footwear, while providing protection against ground reaction forces, may introduce instability that contributes to a tendency for over-pronation. Pronation is the inward roll of the foot while in contact with the ground. Although pronation is normal, it may be a potential source of foot and leg injury, particularly if it is excessive. Stability devices are often incorporated into foam midsoles to control pronation of the foot. Examples of stability devices are found in U.S. Pat. No. 4,255,877 to Bowerman; U.S. Pat. No. 4,287,675 to Norton et al.; U.S. Pat. No. 4,288,929 to Norton et al.; U.S. Pat. No. 4,354,318 to Frederick et al.; U.S. Pat. No. 4,364,188 to Turner et al.; U.S. Pat. No. 4,364,189 to Bates; and U.S. Pat. No. 5,247,742 to Kilgore et al. In addition to increasing the tendency for over-pronation, conventional foam midsoles exhibit localized ground reaction force distributions. That is, foam midsoles often distribute ground reaction forces only to the area immediately adjacent to the point of impact, thereby transferring the ground reaction forces to the portion of the foot located generally above the point of impact. In order to distribute ground reaction forces to a greater portion of the midsole and foot, foam midsoles may incorporate moderators. An example of a moderator is a fluid-filled bladder, as disclosed by U.S. Pat. No. 4,183,156 and U.S. Pat. No. 4,219,945 to Marion F. Rudy.
The present invention relates to an article of footwear having an upper for receiving a foot of a wearer and a sole attached to the upper. The sole is located generally below the foot and includes a three-dimensional, compressible, semi-rigid lattice structure having a plurality of connectors joined by a plurality of masses. The physical and material properties of the connectors and the masses may be configured such that ground reaction forces incident the lattice structure are attenuated and distributed substantially throughout the lattice structure.
The connectors of the lattice structure may be straight, curved, or x-shaped, for example. Similarly, the connectors may have a variety of lengths and cross-sectional shapes. The masses may be generally spherical or may have a variety of other shapes within the scope of the present invention. Accordingly, the lattice structure may be formed of a variety of types of connectors and masses, thereby imparting a variety of lattice structure configurations that each have different properties.
By varying the configuration of the lattice structure, the degree of ground reaction force attenuation, the manner in which ground reaction forces are distributed, and the vibrational frequency of the lattice structure may be selected to achieve a specific purpose. For example, the ground reaction force distribution and vibrational frequency of the lattice structure may be configured to mimic the response of barefoot running, but with the attenuated ground reaction forces. That is, the lattice structure could be designed to impart the feeling of barefoot running, but with a reduced level of ground reaction forces. Additionally, the ground reaction forces could be more concentrated in the medial portion of the foot than in the lateral portion of the foot, thereby imparting greater stability or reducing the probability that the foot will over-pronate.
Although the sole may include a uniform lattice structure that extends from the forefoot area to the heel area, the lattice structure may have a non-uniform structure. Accordingly, the configuration of the connectors and masses may be changed depending upon the area of the foot that each portion of the lattice structure corresponds with. In addition, the lattice structure may be formed of two or more blocks that are separated to prevent vibrations from one block from interfering with the vibrations of an adjacent block.
The lattice structure may be used independent of a conventional outsole such that the lattice structure directly contacts the ground. To reduce wear and provide traction, portions of the lattice structure, such as the masses, may include caps. In addition, a perforated membrane may be used to prevent debris from becoming trapped within interstitial areas of the lattice structure.
The advantages and features of novelty characterizing the present invention are pointed out with particularity in the appended claims. To gain an improved understanding of the advantages and features of novelty, however, reference may be made to the following descriptive matter and accompanying drawings that describe and illustrate various embodiments and concepts related to the invention.